Communication networks in digital substation automation systems have been divided into several layers of networks, i.e. so-called buses. Each layer may carry different types of traffic, pertinent to the functions of the devices connected to this layer. Part of the network traffic can be considered horizontal; i.e., the information carried is only relevant for recipients on this network layer, not on other layers. In general it is desirable, e.g. for performance reasons, to contain such horizontal traffic within the network layer and not forward it to other network layers. However, an increasing amount of traffic is vertical; i.e., it needs to be forwarded to lower or higher network layers. Depending on the type and content of the traffic it may be necessary to forward it as is, filter and forward only parts of a data stream, or aggregate selected content to a new data stream which is then forwarded.
Substation communication networks are often required to implement redundancy schemes according to IEC 62439-3 such as PRP or HSR in order to increase the availability of the communication system. Consequently, the bridging connections between network layers should be redundant too. This poses a technical issue for blocking, filtering and aggregating traffic between network layers, because the redundant bridging connections should operate in tight coordination in order to obey the rules of the redundancy scheme. As a result, the standard behaviour of existing solutions such as redundant quad-boxes is to just forward all traffic between networks layers; filtering and aggregation is not supported.
Nowadays, redundant bridges and gateways are known for communication systems. The redundant bridges are variations of quad-boxes. Their main feature is the redundant forwarding of traffic between network layers that themselves implement a redundancy scheme. A frame is duplicated for forwarding at the sending network and the receiving network discards one of the duplicated frames that arrived later. The quad-boxes can achieve traffic filtering if they support VLANs. However, aggregation of data at the network bridge is not possible. The topology of the source network is not hidden; i.e., quad-boxes “project” one network layer into the other.
The gateways, on the other hand, have the function to filter and potentially translate data between networks or network layers, but do not provide redundancy and consequently do not have zero recovery time in case of failure. The gateways can also introduce a considerable latency when bridging networks.